Appearance of Extrachromosomal Circular DNA in Lymphocytes from Developing Chick Bursa

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Abstract
Lymphocytes were purified from the developing bursa of Fabricius of chick embryos and chickens and pressed with a mica sheet. Then the extruded DNA complexes were adsorbed to the mica and processed for EM. Circular DNA complexes were found at 40-60 copies in all bursal lymphocyte except 12-day-old embryonic bursas, which contained 150 copies. Circular DNA complexes of > 1 .mu.m (larger circular DNA) appeared in 12-day-old embryonic bursas at > 20 copies per cell, and subsequently decreased in number to < 10 copies per cell. These changes in the size distribution and copy number of circular DNA coincided with lymphocyte differentiation of the hemopoietic precursor cells in the bursa after seeding.